1. Field of the Invention
The present invention relates to a method for thawing food in a microwave oven, and more particularly to a food thawing method capable of effectively thawing a small amount of food.
2. Description of the Background Art
Referring to FIG. 1, there is shown a thawing device equipped in a microwave oven. As shown in FIG. 1, the thawing device includes a heating chamber 1 for heating a food 2 disposed therein. A turntable 3 is rotatably disposed in the heating chamber 1. The turntable 3 supports the food 2 thereon. Thawing device also includes a turntable motor 4 for rotating the turntable 3 and an exhaust port 5 for exhausting water vapor and gas generated in the heating chamber 1. A gas sensor 6 is disposed near the exhaust port 5 so as to sense water vapor and gas exhausted through the exhaust port 5. The thawing device further includes a microcomputer 7 for calculating a thawing time for the food 2 based on an output signal from the gas sensor 6 and controlling various parts of the microwave oven, a display unit 8 for displaying the thawing time calculated by the microcomputer 7 and other information such as cooking time, a magnetron 10 for generating a radio frequency wave, an output control unit 9 for controlling driving of the magnetron 10 under a control of the microcomputer 7, and a key input unit 11 for selecting a function desired by a user.
The operation of the thawing device having the abovementioned construction will now be described.
When the user lays the food 2 to be thawed on the turntable 3 disposed in the heating chamber 1 for thawing the food and manipulates the key input unit 11, the microcomputer 7 determines whether an input key signal generated from the key input unit 11 corresponds to an automatic thawing key signal. Where the generated input key signal does not correspond to the automatic thawing key signal, a function according to the input key signal is carried out. However, where the current input key signal corresponds to the automatic thawing key signal, the microcomputer 7 checks a door condition of the microwave oven. When the door is at its closed state, the microcomputer 7 sends a control signal to the output control unit 9. Under the control of the microcomputer 7, the output control unit 9 controls the magnetron 10 to oscillate, so that the magnetron 10 outputs radio frequency waves. That is, the magnetron 10 is controlled to oscillate for 10 seconds and then stop for 12 seconds repeatedly, as shown in FIG. 3.
Now, the procedure of thawing the food will be described in terms of heating time. At an initial thawing step, the radio frequency wave energy generated by the oscillation of the magnetron 10 permeates the food 2, thereby causing the frozen food 2 to be heated, as shown in A of FIG. 4. As the food 2 is heated, the surface of food 2 is thawed, thereby forming a water film, as shown in B of FIG. 4. At this time, the surface temperature of the food 2 is in excess of 0.degree. C., while the internal temperature of the food 2 is uniformly increased, as compared to the state shown in A of FIG. 4. As the food is further heated, moisture and gas are generated from the water film on the surface of food 12, as shown in C of FIG. 4. The generated moisture and gas are exhausted through the exhaust port 5. At this time, the internal temperature of food 2 is increased to a level approximate to 0.degree. C. On the other hand, the magnetron 10 is controlled to output radio frequency wave energy corresponding to 30 to 50% of its maximum output. This output range may be varied depending on the output grade of the microwave range used.
When the water vapor and gas generated from the food 2 being thawed are exhausted through the exhaust port 5, the gas sensor 6 senses them and generates an electrical signal indicative of the result of its sensing. The microcomputer 7 receives the output signal from the gas sensor 16. When the gas sensor 6 sends a signal having a waveform shown in FIG. 5 to the microcomputer 7, the microcomputer 7 derives the resistance ratio of the output signal of gas sensor 6 by the lapse of time, as shown in FIG. 6. FIG. 6 shows graph illustrating the resistance ratio of the output signal of gas sensor 6 by the lapse of time. In FIG. 6, the line A corresponds to a case where the food 2 is small in amount, while the line B corresponds to a case where the food 2 is large in amount. As shown in FIG. 6, an inflexion phenomenon occurs at the point of time when the frozen food is thawed more or less, namely the point of time t1 or t2. This is because absorption of the radio frequency wave energy is rapidly carried out at the portion of food 2 being thawed, thereby accelerating the generation of water vapor or gas. After one of the lines A and B of FIG. 6 is obtained, the microcomputer 7 senses the inflexion point t1 or t2 each indicative of a melting point of the frozen food 2, from the graph. Where the resistance ratio of the output signal of gas sensor 6 is not less than 1.2, the microcomputer 7 operates to end the thawing operation. On the other hand, where the resistance ratio is less than 1.2, the microcomputer 7 operates to execute additional heating with decreased radio frequency wave energy for a predetermined time T2 in order to secondarily thaw the food 2. Upon secondarily heating the food 2 in the interval T2, the magnetron 10 is controlled to oscillate for 4 seconds and then stop for 18 seconds repeatedly. At this time, the gas sensor 6 generates an output signal having a waveform indicated in the interval T2 of FIG. 5.
At the inflexion point, remarkable inflexion may not occur depending on the condition of the food 2 or the surrounding circumstance. In this case, the microcomputer 7 regards the thawing of food 2 to be completed when the output signal from the gas sensor 6 reaches a predetermined value experimentally given, so as to complete the thawing operation.
In accordance with the prior art, however, where a small amount of food is subjected to a thawing treatment meeting a large amount of food, a phenomenon that the food is partially boiled. On the other hand, where a large amount of food is subjected to a thawing treatment meeting a small amount of food, a phenomenon that the food is insufficiently thawed.